Flat display devices have been widely used due to their advantages such as slimness, low power consumption, non-radiation, and the like. A currently available flat display device mainly includes a liquid crystal display device (LCD) and an organic light emitting diode (OLED) display device.
An OLED display device displays an image by self-illumination, and thus does not require any backlight. Accordingly, the OLED display device is regarded as a next generation of display device that may replace the LCD due to its outstanding characteristics such as high contrast ratio, small thickness, wide viewing angle, fast response speed, flexibility, wide operating temperature range, simple structure and manufacturing process, and the like.
The OLED may be classified into two major types of a passive matrix OLED (PMOLED) and an active matrix OLED (AMOLED) according to a driving scheme, which may correspond to two types of a directly addressing type and a thin film transistor (TFT) matrix addressing type. The PMOLED has relative high power consumption and thus is not suitable for large sized display devices. Accordingly, the PMOLED is generally used in small sized display devices. The AMOLED is generally used in large sized display devices with high definition due to its high luminous efficacy.
FIG. 1 illustrates a circuit diagram of a pixel circuit of an AMOLED in the prior art. In a display region of the AMOLED display device, pixels are arranged in a matrix of a plurality of rows and columns, and each pixel is generally driven by a pixel circuit including two TFTs and a capacitor, i.e., each pixel is driven by a 2T1C driving scheme. In particular, a gate electrode of a first transistor T1 is electrically connected to the gate line Scan, a source electrode of the first transistor T1 is electrically connected to the data signal line DATA, and a drain electrode of the first transistor T1 is electrically connected to a gate electrode of a second transistor T2 and one end of a capacitor C. A source electrode of the second transistor T2 is electrically connected to a high voltage signal terminal VDD, and a drain electrode of the second transistor T2 is electrically connected to an anode of an OLED D. A cathode of the OLED D is electrically connected to a common grounding electrode VSS. The one end of the capacitor C is electrically connected to the drain electrode of the first transistor T1, and the other end of the capacitor C is electrically connected to the source electrode of the second transistor T2. While displaying an image, the gate line Scan controls the first transistor T1 to be turned on, and a data signal voltage from the data signal line DATA is provided to the gate electrode of the second transistor T2 and the capacitor C through the first transistor T1. Afterwards, the first transistor T1 is turned off, and a voltage at the gate electrode of the second transistor T2 may be maintained at the data signal voltage due to the capacitor C, such that the second transistor T2 is turned on. Accordingly, a driving current corresponding to the high voltage signal terminal VDD and the data signal voltage may be provided to the OLED D through the second transistor T2, thereby driving the OLED D to emit light.
In the above AMOLED display device, the OLED D is driven according to the current generated under a saturated state of the second transistor T2. However, a critical voltage of the second transistor T2 in each pixel may be different due to the non-uniformity of the TFT manufacturing process. Also, various shifts may occur in the threshold voltage Vth of the second transistor T2 during the illumination of the OLED D. Accordingly, while driving the OLED with the above 2T1C driving circuit, the respective pixels may have a poor brightness uniformity, resulting in a defect such as display unevenness.
It should be noted that, information disclosed in the above background portion is provided only for better understanding of the background of the present disclosure, and thus it may contain information that does not form the prior art known by those ordinary skilled in the art.